Distributed Fiber Deformation Measurement by High-Accuracy Phase Detection in OFDR Scheme

Fiber deformation measurement is crucial in the structure health monitor, which reflects the stress, strain or deformation of the structure where the fiber is laid. The principle of the fiber deformation measurement by the optical phase tracking in OFDR scheme is described and the data processing algorithm is demonstrated in details to receive accurate results. The differential phase is acquired by making a difference between the phase spectrum of the reference and measurement states. A robust phase unwrapping method based on the density distribution is proposed to get the whole unwrapped low-noise phase of the fiber. Additionally, the phase decoherence caused by the scatter unit misalignment induced by the fiber deformation is analyzed and the phase correction method is proposed to improve the phase coherence. The fiber deformation can be tracked along the fiber with a resolution of tens of microns, depending on the set parameters of the OFDR system. Moreover, the strain distribution is also derived from the differential phase. In the experiments, the measurements of the standard single-mode fiber with uniform and non-uniform deformation have been accomplished in a spatial resolution of 40 μm. The noise is about 0.1 μm in a stretching range of about 40 μm. Simultaneously, hundreds of microstrain are obtained and show a high accuracy with a resolution of the level of millimeter.